Metrology method and apparatus, and device manufacturing method

ABSTRACT

Methods are disclosed for measuring target structures formed by a lithographic process on a substrate. A grating or other structure within the target is smaller than an illumination spot and field of view of a measurement optical system. The position of an image of the component structure varies between measurements, and a first type of correction is applied to reduce the influence on the measured intensities, caused by differences in the optical path to and from different positions. A plurality of structures may be imaged simultaneously within the field of view of the optical system, and each corrected for its respective position. The measurements may comprise first and second images of the same target under different modes of illumination and/or imaging, for example in a dark field metrology application. A second type of correction may be applied to reduce the influence of asymmetry between the first and second modes of illumination or imaging, for example to permit a more accurate overly measurement in a semiconductor device manufacturing process.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. 119(e) to U.S.Provisional Application No. 61/412,980, filed Nov. 12, 2010, which isincorporated by reference herein in its entirety.

BACKGROUND

1. Field of Embodiments of the Present Invention

The present invention relates to methods and apparatus for metrologyusable, for example, in the manufacture of devices by lithographictechniques and to methods of manufacturing devices using lithographictechniques.

2. Background Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the patter through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In lithographic processes, it is desirable frequently to makemeasurements of the structures created, e.g., for process control andverification. Various tools for making such measurements are known,including scanning electron microscopes, which are often used to measurecritical dimension (CD), and specialized tools to measure overlay, theaccuracy of alignment of two layers in a device. Recently, various formsof scatterometers have been developed for use in the lithographic field.These devices direct a beam of radiation onto a target and measure oneor more properties of the scattered radiation—e.g., intensity at asingle angle of reflection as a function of wavelength; intensity at oneor more wavelengths as a function of reflected angle; or polarization asa function of reflected angle—to obtain a “spectrum” from which aproperty of interest of the target can be determined. Determination ofthe property of interest may be performed by various techniques: e.g.,reconstruction of the target structure by iterative approaches such asrigorous coupled wave analysis or finite element methods; librarysearches; and principal component analysis.

The targets used by conventional scatterometers are relatively large,e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spotthat is smaller than the grating (i.e., the grating is underfilled).This simplifies mathematical reconstruction of the target as it can beregarded as infinite. However, in order to reduce the size of thetargets, e.g., to 10 μm by 10 μm or less, e.g., so they can bepositioned in amongst product features, rather than in the scribe lane,so-called “small target” metrology has been proposed, in which thegrating is made smaller than the measurement spot (i.e., the grating isoverfilled). Typically small targets are measured using dark fieldscatterometry in which the zeroth order of diffraction (corresponding toa specular reflection) is blocked, and only higher orders processed.Diffraction-based overlay using dark-field detection of the diffractionorders enables overlay measurements on smaller targets. These targetscan be smaller than the illumination spot and may be surrounded byproduct structures on a wafer. The intensities from the environmentproduct structures can efficiently be separated from the intensitiesfrom the overlay target with the dark-field detection in theimage-plane.

In the known dark-field metrology technique, best overlay measurementresults are obtained if the target is measured twice under certainconditions, while the wafer is rotated to obtain both the −1^(st) andthe +1^(st) diffraction order intensities. The use of exactly the sameoptical path for both measurements ensures that the different betweenthem is due to target properties, not properties of the instrument. Onthe other haul, the requirement to rotate the target, which may be on alarge substrate, makes the process slow and the apparatus potentiallymore complex. Also, if the target after rotation is not positionedexactly where it was measured before, it cannot be assumed that theoptical system will perform exactly the same in both measurements.Accurate positioning requires more costly equipment and/or costs time inthe measurement process. The problem of accurate positioning may arisein other types of metrology instruments and methods, besides the darkfield method using scatterometers. For example the same problem mayarise where intensity measurements are made using zero order radiationreflected from targets smaller than the illumination spot.

SUMMARY

It is desirable to provide a method and apparatus for dark fieldmetrology, for example to measure asymmetry and/or overlay in targetgratings, in which throughput and accuracy can be improved over priorpublished techniques.

A first embodiment of the present invention provides a method ofmeasuring asymmetry in a periodic structure formed by a lithographicprocess on a substrate, the method comprising the steps of: using thelithographic process to form a periodic structure on the substrate, afirst measurement step comprising forming and detecting a first image ofthe periodic structure while illuminating the structure with a firstbeam of radiation, the first image being formed using a first part ofnon-zero order diffracted radiation while excluding zero orderdiffracted radiation, a second measurement step comprising forming anddetecting a second image of the periodic structure while illuminatingthe structure with a second beam of radiation, the second image beingformed using a second part of the non-zero order diffracted radiationwhich is symmetrically opposite to the first part, in a diffractionspectrum of the periodic structure, and using a difference in intensityvalues derived from the detected first and second images together todetermine the asymmetry in the profile of the periodic structure. Thefirst and second measurement steps are performed using different opticalpaths within a measurement optical system. The method further comprisesat least one correction step, applied either to the first and secondimage intensity values or to the calculated difference between them, forreducing an influence on the determined asymmetry of the difference inoptical paths between the first and second measurement steps.

The correction may be effective to reduce the influence of one or moreof the following: a change in illumination mode between the first andsecond measurement steps, a change in imaging mode between the first andsecond measurement steps, and the position of the periodic structurewithin an image field of the optical system in each of the first andsecond measurement steps.

Embodiments of the present invention may be used in small target, darkfield overlay metrology. The determined asymmetry may be used as ameasure of overlay in a multi-layered grating target, so that feedbackcan be applied to a lithographic process to reduce overlay error insubsequent patterning operations.

A second embodiment of the present invention further provides aninspection apparatus configured for measuring asymmetry in a periodicstructure on a substrate, the inspection apparatus comprising: anillumination arrangement operable to deliver first and second beams ofradiation to the substrate for use in first and second measurementsteps, a detection arrangement operable during the first and secondmeasurement steps to form and detect respective first and second imagesof the substrate using radiation diffracted from the substrate, and astop arrangement within the detection arrangement. The illuminationarrangement and stop arrangement together are effective to stop zeroorder diffracted radiation contributing to the first and second images,and are configurable to form first and second images using first andsecond parts respectively of the non-zero order diffracted radiation,the first and second parts being symmetrically opposite one another in adiffraction spectrum of the diffracted radiation, wherein the inspectionapparatus further comprises a computational arrangement operable todetermine the asymmetry using a difference in intensity values derivedfrom the first and second images, and wherein the computationalarrangement is operable when calculating the difference to apply acorrection for reducing an influence on the determined asymmetry of adifference between first and second optical paths, that are used withinthe inspection apparatus for the first and second measurement stepsrespectively.

A third embodiment of the present invention provides a method ofmeasuring properties of a target structure formed by a lithographicprocess on a substrate, the method comprising the steps of: using thelithographic process to form a structure on the substrate, forming anddetecting an image of the structure through an optical system whileilluminating the structure with a beam of radiation, and using intensityvalues derived from the detected image together to determine at leastone parameter of the structure. The image of the structure is smallerthan an image field of the optical system. The method further comprisesdetecting a position of the image within the image field and applying acorrection to reduce an influence on the measured properties of adifference in optical paths between positions.

Embodiments of the present invention may be applied in small-targetmetrology more broadly than dark field metrology.

Another embodiments of the present invention provides an inspectionapparatus for measuring properties of a target structure formed by alithographic process on a substrate, the inspection apparatuscomprising: an illumination arrangement operable to deliver a radiationbeam to the substrate, a detection arrangement operable to form anddetect an image of the structure through an optical system while theillumination arrangement is illuminating the structure with a radiationbeam, a computational arrangement operable: to use intensity valuesderived from the detected image together to determine at least oneproperty of the structure, to detect a position of the image within theimage field when the image of the structure is smaller than an imagefield of the optical system, in response to detected position, to applya correction to reduce an influence on the measured property ofdifferences in optical paths associated with different image positions.

A still further embodiment of the present invention provides alithographic system including a lithographic apparatus and an inspectionapparatus according to the first and/or second aspect of Embodiments ofthe present invention, as set forth above.

An even further embodiment of the present invention provides a method ofmanufacturing devices wherein a device pattern is applied to a series ofsubstrates using a lithographic process, the method including inspectingat least one periodic structure formed as part of or beside the devicepattern on at least one of the substrates using an inspection methodaccording to the embodiments of the present invention as set forthabove, and controlling the lithographic process for later substrates inaccordance with the result of the inspection method.

Further features and advantages of embodiments of the present invention,as well as the structure and operation of various embodiments ofembodiments of the present invention, are described in detail below withreference to the accompanying drawings. It is noted that embodiments ofthe present invention is not limited to the specific embodimentsdescribed herein. Such embodiments are presented herein for illustrativepurposes only. Additional embodiments will be apparent to personsskilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles ofembodiments of the present invention and to enable a person skilled inthe relevant art(s) to make and use Embodiments of the presentinvention.

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe present invention.

FIG. 2 depicts a lithographic cell or cluster according to an embodimentof the present invention.

FIG. 3 comprises (a) a schematic diagram of a dark field scatterometerfor use in measuring targets according to embodiments of Embodiments ofthe present invention using a first pair of illumination apertures, (b)a detail of diffraction spectrum of a target grating for a givendirection of illumination (c) a second pair of illumination aperturesproviding further illumination modes in using the scatterometer fordiffraction based overlay measurements and (d) a third pair ofillumination apertures combining the first and second pair of apertures.

FIG. 4 depicts a known form of target and an outline of a measurementspot on a substrate.

FIG. 5 depicts an image of the targets of FIG. 4 obtained in thescatterometer of FIG. 3.

FIG. 6 is a flowchart showing the steps of an overlay measurement usingthe scatterometer of FIG. 3.

FIG. 7 depicts (a) first and (b) second images of the targets obtainedusing first and second illumination modes in the process of FIG. 6.

FIG. 8 depicts (a) first and (b) second images of the targets obtainedusing first and second illumination modes in the process of FIG. 6,where positioning of the targets in an image field is subject tovariation.

FIG. 9 is a flowchart showing the steps of an overlay measurementprocess incorporating correction steps in accordance with the presentinvention.

FIG. 10 illustrates an alternative form of target in (a) plan view and(b) schematic cross-section.

FIG. 11 illustrates a set of measurements made using the target of FIG.10 in a calibration step of a first implementation of the presentinvention.

FIG. 12 illustrates a calibration and correction process in a secondimplementation of an embodiment of the present invention.

FIG. 13 illustrates a calibration and correction process in a thirdimplementation of embodiments of the present invention.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The drawing in which an elementfirst appears is indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION

This specification discloses one or more embodiments that incorporatethe features of this invention. The disclosed embodiment(s) merelyexemplify Embodiments of the present invention. The scope of embodimentsof the present invention is not limited to the disclosed embodiment(s).Embodiments of the present invention are defined by the claims appendedhereto.

The embodiment(s) described, and references in the specification to “oneembodiment”, “an embodiment”, “an example embodiment”, etc, indicatethat the embodiment(s) described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or character characteristic in connection with otherembodiments whether or not explicitly described.

Embodiments of the present invention may be implemented in hardware,firmware, software, or any combination thereof. Embodiments of thepresent invention may also be implemented as instructions stored on amachine-readable medium, which may be read and executed by one or moreprocessors. A machine-readable medium may include any mechanism forstoring or transmitting information in a form readable by a machine(e.g., a computing device). For example, a machine-readable medium mayinclude read only memory (ROM); random access memory (RAM); magneticdisk storage media; optical storage media; flash memory devices;electrical, optical, acoustical or other forms of propagated signals(e.g., carrier waves, infrared signals, digital signals, etc.), andothers. Further, firmware, software, routines, instructions may bedescribed herein as performing certain actions. However, it should beappreciated that such descriptions are merely for convenience and thatsuch actions in fact result from computing devices, processors,controllers, or other devices executing the firmware, software,routines, instructions, etc.

Before describing embodiments of the present invention in detail, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g., UV radiation or DUV radiation), a patterningdevice support or support structure (e.g., a mask table) MT constructedto support a patterning device (e.g., a mask) MA and connected to afirst positioner PM configured to accurately position the patterningdevice in accordance with certain parameters; a substrate table (e.g., awafer table) WT constructed to hold a substrate (e.g., a resist coatedwafer) W and connected to a second positioner PW configured toaccurately position the substrate in accordance with certain parameters;and a projection system (e.g., a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g., including one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support may be a frame or a table, for example, whichmay be fixed or movable as required. The patterning device support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support (e.g., mask tableMT), and is patterned by the patterning device. Having traversed thepatterning device (e.g., mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor IF (e.g., an interferometric device, linear encoder, 2-Dencoder or capacitive sensor), the substrate table WT can be movedaccurately, e.g., so as to position different target portions C in thepath of the radiation beam B. Similarly, the first positioner PM andanother position sensor (which is not explicitly depicted in FIG. 1) canbe used to accurately position the patterning device (e.g., mask) MAwith respect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe patterning device support (e.g., mask table) MT may be realized withthe aid of a long-stroke module (coarse positioning) and a short-strokemodule (fine positioning), which form part of the first positioner PM.Similarly, movement of the substrate table WT may be realized using along-stroke module and a short-stroke module, which form part of thesecond positioner PW. In the case of a stepper (as opposed to a scanner)the patterning device support (e.g., mask table) MT may be connected toa short-stroke actuator only, or may be fixed.

Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g., mask) MA, the mask alignment marks may be located betweenthe dies. Small alignment markers may also be included within dies, inamongst the device features, in which case it is desirable that themarkers be as small as possible and not require any different imaging orprocess conditions than adjacent features. The alignment system, whichdetects the alignment markers is described further below.

The depicted apparatus could be used in at least one of the followingmodes:

-   1. In step mode, the patterning device support (e.g., mask table) MT    and the substrate table WT are kept essentially stationary, while an    entire pattern imparted to the radiation beam is projected onto a    target portion C at one time (i.e., a single static exposure). The    substrate table WT is then shifted in the X and/or Y direction so    that a different target portion C can be exposed. In step mode, the    maximum size of the exposure field limits the size of the target    portion C imaged in a single static exposure.-   2. In scan mode, the patterning device support (e.g., mask table) MT    and the substrate table WT are scanned synchronously while a pattern    imparted to the radiation beam is projected onto a target portion C    (i.e., a single dynamic exposure). The velocity and direction of the    substrate table WT relative to the patterning device support (e.g.,    mask table) MT may be determined by the (de-)magnification and image    reversal characteristics of the projection system PS. In scan mode,    the maximum size of the exposure field limits the width (in the    non-scanning direction) of the target portion in a single dynamic    exposure, whereas the length of the scanning motion determines the    height (in the scanning direction) of the target portion.-   3. In another mode, the patterning device support (e.g., mask table)    MT is kept essentially stationary holding a programmable patterning    device, and the substrate table WT is moved or scanned while a    pattern imparted to the radiation beam is projected onto a target    portion C. In this mode, generally a pulsed radiation source is    employed and the programmable patterning device is updated as    required after each movement of the substrate table WT or in between    successive radiation pulses during a scan. This mode of operation    can be readily applied to maskless lithography that utilizes    programmable patterning device, such as a programmable mirror array    of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Lithographic apparatus LA is of a so-called dual stage type which hastwo substrate tables WTa, WTb and two stations—an exposure station and ameasurement station—between which the substrate tables can be exchanged.While one substrate on one substrate table is being exposed at theexposure station, another substrate can be loaded onto the othersubstrate table at the measurement station and various preparatory stepscarried out. The preparatory steps may include mapping the surfacecontrol of the substrate using a level sensor LS and measuring theposition of alignment markers on the substrate using an alignment sensorAS. This enables a substantial increase in the throughput of theapparatus. If the position sensor IF is not capable of measuring theposition of the substrate table while it is at the measurement stationas well as at the exposure station, a second position sensor may beprovided to enable the positions of the substrate table to be tracked atboth stations.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include spin coaters SCto deposit resist layers, developers DE to develop exposed resist, chillplates CH and bake plates BK. A substrate handler, or robot, RO picks upsubstrates from input/output ports I/O1, I/O2, moves them between thedifferent process apparatus and delivers then to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

Examples of dark field metrology can be found in international patentapplications WO 2009/078708 and WO 2009/106279, which documents arehereby incorporated by reference in their entirety.

A dark field metrology apparatus according to an embodiment of thepresent invention is shown in FIG. 3( a). A target grating T anddiffracted rays are illustrated in more detail in FIG. 3( b). The darkfield metrology apparatus may be a stand-alone device or incorporated ineither the lithographic apparatus LA, e.g., at the measurement station,or the lithographic cell LC. An optical axis, which has several branchesthroughout the apparatus, is represented by a dotted line O. In thisapparatus, light emitted by source 11 (e.g., a xenon lamp) is directedonto substrate W via a beam splitter 15 by an optical system comprisinglenses 12, 14 and objective lens 16. These lenses are arranged in adouble sequence of a 4F arrangement. Therefore, the angular range atwhich the radiation is incident on the substrate can be selected bydefining a spatial intensity distribution in a plane that presents thespatial spectrum of the substrate plane, here referred to as a(conjugate) pupil plane. In particular, this can be done by inserting anaperture plate 13 of suitable form between lenses 12 and 14, in a planewhich is a back-projected image of the objective lens pupil plane. Inthe example illustrated, aperture plate 13 has different forms, labeled13N and 13S, allowing different illumination modes to be selected. Theillumination system in the present examples forms an off-axisillumination mode. In the first illumination mode, aperture plate 13Nprovides off-axis from a direction designated, for the sake ofdescription only, as ‘north’. In a second illumination mode, apertureplate 13S is used to provide similar illumination, but from an oppositedirection, labeled ‘south’. Other modes of illumination are possible byusing different apertures. The rest of the pupil plane is desirably darkas any unnecessary light outside the desired illumination mode willinterfere with the desired measurement signals.

As shown in FIG. 3( b), target grating T is placed with substrate Wnormal to the optical axis O of objective lens 16. A ray of illuminationI impinging on grating T from an angle off the axis O gives rise to azeroth order ray (solid line 0) and two first order rays (dot-chain line+1 and double dot-chain line −1). It should be remembered that with anoverfilled small target grating, these rays are just one of manyparallel rays covering the area of the substrate including metrologytarget grating T and other features. Since the aperture in plate 13 hasa finite width (necessary to admit a useful quantity of light, theincident rays I will in fact occupy a range of angles, and thediffracted rays 0 and +1/−1 will be spread out somewhat. According tothe point spread function of a small target, each order +1 and −1 willbe further spread over a range of angles, not a single ideal ray asshown. Note that the grating pitches and illumination angles can bedesigned or adjusted so that the first order rays entering the objectivelens are closely aligned with the central optical axis. The raysillustrated in FIGS. 3( a) and 3(b) are shown somewhat off axis, purelyto enable them to be more easily distinguished in the diagram.

At least the 0 and +1 orders diffracted by the target on substrate W arecollected by objective lens 16 and directed back through beam splitter15. Returning to FIG. 3( a), both the first and second illuminationmodes are illustrated, by designating diametrically opposite apertureslabeled as north (N) and south (S). When the incident ray I is from thenorth side of the optical axis, that is when the first illumination modeis applied using aperture plate 13N, the +1 diffracted rays, which arelabeled +1(N), enter the objective lens 16. In contrast, when the secondillumination mode is applied using aperture plate 13S to −1 diffractedrays (labeled −1(S) are the ones which enter the lens 16.

A second beam splitter 17 divides the diffracted beams into twomeasurement branches. In a first measurement branch, optical system 18forms a diffraction spectrum (pupil plane image) of the target on firstsensor 19 (e.g., a CCD or CMOS sensor) using the zeroth and first orderdiffractive beams. Each diffraction order hits a different point on thesensor, so that image processing can compare and contrast orders. Thepupil plane image captured by sensor 19 can be used for focusing themetrology apparatus and/or normalizing intensity measurements of thefirst order beam. The pupil plane image can also be used for manymeasurement purposes such as reconstruction, which are not the subjectof the present disclosure.

In the second measurement branch, optical system 20, 22 forms an imageof the target on the substrate W on sensor 23 (e.g., a CCD or CMOSsensor). In the second measurement branch, an aperture stop 21 isprovided in a plane that is conjugate to the pupil-plane. Aperture stop21 functions to block the zeroth order diffracted beam so that the imageof the target formed on sensor 23 is formed only from the −1 or +1 firstorder beam. The images captured by sensors 19 and 23 are output to imageprocessor and controller PU, the function of which will depend on theparticular type of measurements being performed. Note that the term‘image’ is used here in a broad sense. An image of the grating lines assuch will not be formed, if only one of the −1 and +1 orders is present.

The particular forms of aperture plate 13 and field stop 21 shown inFIG. 3 are purely examples. In another embodiments of the presentinvention, on-axis illumination of the targets is used and an aperturestop with an off-axis aperture is used to pass substantially only onefirst order of diffracted light to the sensor. In yet other embodiments,2nd, 3rd and higher order beams (not shown in FIG. 3) can be used inmeasurements, instead of or in addition to the first order beams.

In order to make the illumination adaptable to these different types ofmeasurement, the aperture plate 13 may comprise a number of aperturepatterns formed around a disc, which rotates to bring a desired patterninto place. Alternatively or in addition, a set of plates 13 could beprovided and swapped, to achieve the same effect. A programmableillumination device such as a deformable mirror array or transmissivespatial sight modulator can be used also. Moving mirrors or prisms canbe used as another way to adjust the illumination mode.

As just explained in relation to aperture plate 13, the selection ofdiffraction orders for imaging can alternatively be achieved by alteringthe field stop 21, or by substituting a field stop having a differentpattern, or by replacing the fixed field stop with a programmablespatial light modulator. In that case the illumination side of themeasurement optical system can remain constant, while it is the imagingside that has first and second modes. In the present disclosure,therefore, there are effectively two types of embodiment: one where theillumination mode is changed and another where the imaging mode ischanged. In each case the desired effect is the same, namely to selectfirst and second portions of the non-zero order diffracted radiationwhich are symmetrically opposite one another in the diffraction spectrumof the target. For the most part, this description will describeembodiments of the first type, in which illumination modes are changed.Where it is desired to make a distinction between the two types ofembodiment, this will be made clear by reference to “illumination modes”and “imaging modes”. Where no distinction is made, the reader shouldunderstand that the treatment of different illumination modes in theillustrated embodiment can be adapted readily to the imaging modes inthe other type of embodiment. In principle, the desired selection oforders could be obtained by a combination of changing the illuminationmodes and the imaging modes simultaneously, but that is likely to bringdisadvantages for no advantage, so it will not be discussed further.

While the optical system used for imaging in the present examples has awide entrance pupil which is restricted by the field stop 21, in otherembodiments or applications the entrance pupil size of the imagingsystem itself may be small enough to restrict to the desired order, andthus serve also as the field stop. Different aperture plates are shownin FIGS. 3( c) and (d) which can be used as described further below. Forthe time being, it is sufficient to consider simply that the apertureplate 13N is used.

FIG. 4 depicts a composite target formed on a substrate according toknown practice. The composite target comprises four gratings 32 to 35positioned closely together so that they will all be within ameasurement spot 31 formed by the illumination beam of the metrologyapparatus and thus are all simultaneously illuminated and simultaneouslyimaged on sensors 19 and 23. In an example dedicated to overlaymeasurement, gratings 32 to 35 are themselves composite gratings formedby overlying gratings that are patterned in different layers of thesemi-conductor device formed on substrate W. Gratings 32 to 35 aredifferently biased in order to facilitate measurement of overlay betweenthe layers in which the different parts of the composite gratings areformed. In one example, gratings 32 to 35 have biases of +d, −d, +3d,−3d respectively. This means that one of the gratings has its componentsarranged so that if they were both printed exactly at their nominallocations one of the components would be offset relative to the other bya distance d. A second grating has its components arranged so that ifperfectly printed there would be an offset of d but in the oppositedirection to the first grating and so on. While four gratings areillustrated, a practical embodiment might require a larger matrix toobtain the desired accuracy. For example, a 3×3 array of nine compositegratings may have biases −4d, −3d, −2d, −d, 0, +d, +2d, +3d, +4d.Separate images of these gratings can be identified in the imagecaptured by sensor 23.

FIG. 5 shows an example of an image that may be formed on and detectedby the sensor 23, using the target of FIG. 4 in the apparatus of FIG. 3,using the aperture plates 13N or 13S from FIG. 3( a). While the pupilplane image sensor 19 cannot resolve the different individual gratings32 to 35, the image sensor 23 can do so. The dark rectangle representsthe field of the image on the sensor, within which the illuminated spot31 on the substrate is imaged into a corresponding circular area 41.Within this, rectangular areas 42-45 represent the images of the smalltarget gratings 32 to 35. If the gratings are located in product areas,product features may also be visible in this image. Image processor andcontroller PU processes these images to identify the separate images 42to 45 of gratings 32 to 35. This can be done by pattern matchingtechniques, so that the images do not have to be aligned very preciselyat a specific location within the sensor frame. Reducing the need foraccurate alignment in this way greatly improves throughput of themeasuring apparatus as a whole. However the need for accurate alignmentremains if the imaging process is subject to non-uniformities across theimage field. In one embodiments of the present invention, four positionsP1 to P4 are identified and the gratings are aligned as much as possiblewith these known positions.

Once the separate images of the gratings have been identified, theintensities of those individual images can be measured, e.g., byaveraging or summing selected pixel intensity values within theidentified areas Intensities and/or other properties of the images canbe compared with one another. These results can be combined to measuredifferent parameters of the lithographic process. Overlay performance isan important example of such a parameter.

FIGS. 6 and 7 illustrate how, using for example the method described inapplication PCT/EP2010/060894, which is incorporated by reference hereinin its entirety, overlay error between the two layers containing thecomponent gratings 32 to 35 is measured through asymmetry of thegratings, as revealed by comparing their intensities in the +1 order and−1 order dark field images. At step S1, the substrate, for example asemiconductor wafer, is processed through the lithographic cell of FIG.2 one or more times, to create a structure including the overlay targets32-35. At S2, using the metrology apparatus of FIG. 3 with only a singlepole of illumination (e.g., north, using plate 13N), an image of thegratings 32 to 35 is obtained using only one of the first orderdiffracted beams (say −1). Then, according to the prior application,substrate W is rotated by 180° and the gratings repositioned in thefield of view of the metrology apparatus so that a second image of thegratings using the other first order diffracted beam can be obtained(step S3). Consequently the +1 diffracted radiation is captured in thesecond image.

FIG. 7 shows the two images (a) and (b) obtained in steps S2 and S3respectively. The 180° rotation is indicated by the legends RZ=0 andRZ=π, referring to rotation about the Z axis, normal to the substratesurface. The image in FIG. 7( a) looks like that shown in FIG. 5, wherethe area of the illumination spot 31 is imaged using just the −1 orderdiffracted radiation into circle 41(−) and the individual grating imagesare labeled 42(−) to 45(−). FIG. 7( b) looks similar but with thegrating images 42(+) to 45(+) begin made using only the +1 orderradiation, their intensities are different. Also, the 180° rotation hasplaced each grating image at a different one of the positions P1-P4.Note that, by including only half of the first order diffractedradiation in each image, the ‘images’ referred to here are notconventional dark field microscopy images. The individual grating lineswill not be resolved. Each grating will be represented simply by an areaof a certain grey level. The asymmetry of the grating structure, andhence overlay error, can then be determined by the image processor andcontroller PU by (S4) comparing the intensity values obtained for +1 and−1 orders for each grating 32-35 to identify any difference in theirintensity, and (S5) from knowledge of the overlay biases of the gratingsto determine overlay error in the vicinity of the target T.

Typically, a target grating will be aligned with its grating linesrunning either north-south or east-west. That is to say, a grating willbe aligned in the X direction or the Y direction of the substrate W.Note that aperture plate 13N or 13S can only be used to measure gratingsoriented in one direction (X or Y depending on the set-up). Formeasurement of an orthogonal grating, rotation through 90° and 270°might be implemented. More conveniently, however, illumination from eastor west is provided in the illumination optics, using the aperture plate13E or 13W, shown in FIG. 3( c). The aperture plates 13N to 13W can beseparately formed and interchanged, or they may be a single apertureplate which can be rotated by 90, 180 or 270 degrees. As mentionedalready, the off-axis apertures illustrated in FIG. 3( c) could beprovided in field stop 21 instead of in illumination aperture plate 13.In that case, the illumination would be on axis.

FIG. 3( d) shows a third pair of aperture plates that can be used tocombine the illumination modes of the first and second pairs. Apertureplate 13NW has apertures at north and east, while aperture plate 13 SEhas apertures at south and west. One of these gratings will diffractlight from the east and west portions of the aperture plates, while theother grating will diffract light from the north and south portions.Provided that cross-talk between these different diffraction signals isnot too great, measurements of both X and Y gratings can be performedwithout changing the illumination mode.

Because, in the example process just described, the target rotates by180° while the optical system in principle remains constant, anydifferences in intensity between the images made using −1 and +1 ordershould be attributable entirely to asymmetry, and hence overlay error,in the target gratings. However, in practice, a number of issues arisewhich make the intensity difference somewhat dependent on the measuringapparatus, and hence make the overlay or other measurement result lessaccurate.

One of these issues is position-dependence of the measurements. In orderto ensure that the optical system performance is constant between themeasurements in steps S2 and S3, the individual gratings should bepositioned at exactly the same point in the illumination spot, and atthe same position in the sensor image field. Otherwise, there is thepossibility that differences between the measurements are due toinhomogeneous illumination across the illumination spot, and/orvariation in response of sensor 23 across the image field. Thereforemeasuring, for example, grating 32 at position P1 in step S2 andposition P4 in step S3 means that the difference between grating images44(−) and 44(+) potentially includes position-dependent effects. Ofcourse, it would be an option to place each individual grating at acommon, central position in the image field. However, part of theincentive to use such small targets is to allow several gratings to beimaged in one measurement, and the throughput penalty would be severe.

Another issue is that the requirement to rotate the substrate through180°. This complicates the apparatus. The rotation step betweenmeasurements introduces delays which can reduce the throughput of theprocess and increases the risk that measurement conditions between thetwo images will be non-identical.

To avoid the need for the substrate to be rotated, the illumination modecan be changed in the apparatus, while the substrate stays still. Forthis purpose, switching the aperture plates in their pairs 13N/13S or13E/13W or 13NW/13SE allows the 180° to be simulated rather than real.For example, in the measurement steps S2 and S3, the aperture plates 13Sto 13N respectively may be used, while keeping the optical systemotherwise the same. Consequently the −1(S) diffracted radiation iscaptured in the first image and the +1(N) radiation is captured in thesecond image. As mentioned above, the change of illumination mode can beimplemented in other ways, for example using deformable mirror devicesin place of exchangeable aperture plates. The present descriptionincludes all these alternatives as possibilities, but for simplicity theswapping of aperture plates will be used as the representativeimplementation.

Where a number of targets are to be measured across the substrate, thereare various possibilities for sequencing the measurement to achieve themaximum throughput and accuracy with a given hardware set-up. Forexample, measurement steps S2 and S3 can be performed each target,swapping the illumination mode, before moving to the next target. Tominimize the swapping operations, the steps may be performed in theorder S2, S3 for one target, and in the order S3, S2 for the next.Alternatively, the step S2 may be performed for all targets on thesubstrate, or for a certain group of targets, before swapping theillumination mode and performing step S3 for all the targets. The imagescan be stored in unit PU, or external storage. Provided they are indexedor labeled with their corresponding target ID and illumination mode, theorder in which they are obtained is unimportant for their subsequentprocessing.

FIG. 8 shows the two images (a) and (b) obtained in steps S2 and S3respectively. The 180° rotation is still indicated by the legends RZ=0and RZ=π, but bearing in mind that the rotation is by changing anaperture position within the instrument, instead of rotating thesubstrate. As would be expected, the sensor image in FIG. 8( a) looksthe same like that shown in FIG. 5 and FIG. 7( a), except that theeffect of positioning inaccuracy has placed the individual gratingimages 42(−) to 45(−) at positions P1′ to P4′ which are not quite thesame as the ‘ideal’ positions P1 to P4. FIG. 7( b) looks similar butwith different intensities of the grating images 42(+) to 45(+). Notethat the overall arrangement of the gratings in the image field has notchanged, in contrast with FIG. 7( b). However, the positions P1″ to P4″of the individual grating images are potentially different again fromthe positions P1′ to P4′. In the case where the two images are taken byswapping the illumination mode without moving the substrate, positionsP1′ and P1″ are likely to be the same. In general, however, they may bedifferent, as shown.

Also shown in FIG. 8 is the indexing of pixel position in the sensor 23image field. For the sake of example, index i represents the pixelposition in a horizontal direction, while j represents the pixelposition in a vertical direction. These directions may convenientlycorrespond to the X and Y directions on the substrate, but this is notnecessarily so.

If either or both of the illumination spot and the imaging performanceof the scatterometer are somehow different between the illuminationmodes used in step S2 and S3, then the differences calculated in step S4will not represent purely the asymmetry of the target. Such differencesin performance can arise at the illumination side, where the apertureplates 13N, 13S (or other means for delivering the desired illuminationdistribution) are not exactly symmetrical. There may also be asymmetryin the optical components such as lenses and field stop 21, or in sensor23 itself. Such asymmetry can arise not only by manufacturingtolerances, but also after manufacture, by contamination or damage ofoptical surfaces.

Similarly, the illumination source 11 may be such that the intensityand/or phase of illumination spot 31 is not uniform. Therefore, even forthe same grating and illumination/imaging mode, a different intensitywill be measured at each position P1 to P4, reducing the accuracy of theasymmetry measured. In a prior patent application U.S. application Ser.No. 12/855,394, which is incorporated by reference herein in itsentirety, separate calibration curves are used in step S4 to deriveasymmetry/overlay from the difference image, the appropriate curve beingselected according to position at which the measurements were made. Partof the incentive to use the image sensor 23 is to reduce requirementsfor accurate positioning, as already mentioned. Curves cannotnecessarily be derived and stored for every possible permutation ofpositions P1′& P1″, P2′& P2″ etc.

To address these error sources and to provide a more accuratemeasurement method with potentially higher throughput, the presentinventors propose a variety of correction schemes that includecorrections for (i) the use of different light paths to obtain both−1^(st) and +1^(st) diffraction orders, instead of rotation of the waferand (ii) the spot profile variations of the intensity in theillumination spot. Each of these corrections will potentially also ‘mopup’ minor contributions from other sources, such as contamination ordamage, provided those sources are present at the time of calibration.If one of these corrections is not necessary to achieve the desiredperformance, then it can be omitted without departing from the scope ofEmbodiments of the present invention.

With these corrections, throughput can be increased by using oppositelyincident light instead of the time consuming wafer rotation plusnecessary alignment steps. Furthermore, throughput is increased bysimultaneous measurement of several gratings within one target, whichconsequently are spatially separated. They will therefore be positionedat different positions within the spot. Moreover, the sensitivity of themeasurement to exact positioning of the target within the illuminationspot is reduced by the proposed corrections, as well as the sensitivityfor artifacts at the camera like dirt at an equivalent wafer plane.Three example implementations will be described. In the first and secondimplementations, the proposed correction scheme is applied to theregion-of-interest (ROI) of the gratings as a whole. In the secondimplementation, the correction is applied pixel-by-pixel for the wholeillumination spot.

It would also be a problem if the illumination spot were notconsistently positioned within the image field, but in practice thatsource of error can be eliminated by good design.

Before describing specific implementations, FIG. 9 shows the generalprinciples of a modification of the FIG. 6 process, which accommodates acorrection scheme in accordance with the present invention. In a newstep S0, calibration measurements are performed to collect data on theasymmetries and position variations present between modes. Basically,intensity measurements are made in each illumination mode usingidentical targets and target positions, to identify the intensitydifference which is attributable to the change of illumination.Separately, measurement of the same target can be made at differentpositions, without changing the illumination mode. These sets ofmeasurements can be performed in combination. An example will bedescribed later with reference to FIGS. 10 & 11.

Following the measurement in step S2 and prior to the calculation ofintensity difference in step S4, a correction is applied at S2 a toremove characteristics of the first illumination mode that have beendetermined in step S0. A second correction is applied at S2 b to removethe influence of the particular position in the image field where thegrating in question is found (FIG. 7( a) or 8(a)). Following themeasurement in step 3 and prior to the calculation of intensitydifference in step S4, a correction is applied at S3 a to removecharacteristics of the second illumination mode that have beendetermined in step S0. A second correction is applied at S3 b to removethe influence of the particular position in the image field where thegrating in question is found in the second image (FIG. 7( b) or 8(b)).It will be noted that in the embodiments described herein, thecalibration and correction are performed in the domain of measuredintensities, prior to or as part of measuring the intensity differencein step S4. This is different to schemes which would apply a correctionin the later stage of deriving overlay results from the intensitydifference. In principle, it would be possible to use one of theillumination modes as the ‘baseline’ reference for correction of theother. Then, in principle, step S2 a or S3 a could be omitted.Similarly, if one of the positions were adopted as the referenceposition, one of the steps S2 b or S3 b could be omitted. In practice,however, it is likely that each intensity measurement will be correctedwith reference to a common reference. This is particularly desirablewhen both position correction and mode correction are to be implemented,because the two sources of asymmetry become coupled together and tochoose one particular mode and position as a reference would beartificial and inconvenient. Since an absolute performance reference isnot likely to be available, the embodiments described below choose touse the average performance of a pair or set of measurements to serve asthe baseline for the correction of each individual measurement in theset. Since all the measurements are concerned with relative intensityvalues, it does not matter that there is no absolute reference.

Correction Implementation 1:

In a first implementation, an illumination mode correction (steps S2 aand S3 a) corrects for the case that diametrically opposed illuminationapertures are used for the overlay measurement (leading totool-induced-shift (TIS) in the intensities). A spot profile correction(steps S2 b and S3 b) corrects for the case of two or more gratingswithin one target, of which the images are recorded in a single shot.For this case the gratings have necessarily not the same position withinthe illumination spot, and the illumination spot profile (and otherfactors) will influence their intensities. Note that the spot profilecorrection of steps S2 b and S3 b can be applied even in embodimentswhere the substrate is rotated without changing the illumination mode,and the single measured grating is positioned exactly at the sameposition within the spot each time; then steps S2 a and S3 a areunnecessary. However, the availability of the illumination modecorrection opens the possibility to enhance throughput by measuring onlyat 0° wafer rotation.

The skilled person will appreciate that the calibrations andmeasurements in a real application will normally be duplicated for thedifferent orientations of grating, and possibly for different types ofgrating also. References to ‘left’ and ‘right’ in this section can beinterpreted as either ‘north’ and ‘south’ or ‘east’ and ‘west’ (in theterminology of the above description), according to the orientation ofthe grating being measured.

In the first implementation, where there are only two possible positionsfor each grating and only two illumination modes, three correctionparameters are introduced: one for the illumination asymmetry forincident light from left and right, one for the spot profile dependence,and one to describe their interdependence. The following notation istaken: δ describes the average asymmetry between first and secondillumination modes, Δδ indicates the deviation from this averageasymmetry between first and second positions of a grafting, and εdescribes the difference between the average illuminations at thedifferent positions.

The skilled reader will appreciate that different definitions ofcorrection parameters could be chosen, and the calculations adaptedaccordingly. More general examples will be shown in the second and thirdimplementations described below. In an embodiment where the sameposition is always assured, only the first parameter would be required.Naturally, in embodiments where the illumination is constant and thefield stop or equivalent is changed to change the imaging mode, thefirst parameter is adapted to describe asymmetry between imaging modes,rather than illumination modes.

In the equations and diagrams that follow, subscripts QL and QR will beused to refer to measurements made under the left and right illuminationmodes, respectively. In this first implementation, calibration andcorrection is done assuming that the grating images will occupy one oftwo known positions within the image field. Subscripts 1 and 2 will beused to refer to two different grating positions within the spot, forexample positions P1 and P2 of the images in FIGS. 5, 7 and 8, which donot rotate with wafer rotation. The superscripts 0 and π refer to thesubstrate orientation of 0° and 180°, respectively. (Even if thesubstrate is not rotated in the measurement process, a calibrationsubstrate will be rotated for the purposes of calibration.) Theintensities with tilde Ĩ₊₁ and Ĩ⁻¹ refer to the ideal +1 and −1diffraction order intensities from the grating. The grating doesn't haveto be symmetric: Ĩ₊₁ and Ĩ⁻¹, for a biased grating (or a grating withoverlay error) will differ.

To be able to determine the correction parameters δ, Δδ and ε, acalibration measurement (step S0) needs to be performed accurately. Realmeasurements on a single grating can be made at different positions,orientations, and illumination modes. The skilled reader will appreciatehow to extend the calibration and correction parameters to othersituations, for example where more than two positions are possible.Using the notation introduced above, this single grating then yields theeight measured intensities:I _(QL,1) ⁰=(1−(δ+Δδ))(1−ε)Ĩ ₊₁ I _(QL,1) ^(π)=(1−(δ+Δδ))(1−ε)Ĩ ⁻¹I _(QR,1) ⁰=(1+(δ+Δδ))(1−ε)Ĩ ⁻¹ I _(QR,1) ^(π)=(1+(δ+Δδ))(1−ε)Ĩ ₊₁andI _(QL,2) ⁰=(1−(δ−Δδ))(1+ε)Ĩ ₊₁ I _(QL,1) ^(π)=(1−(δ−Δδ))(1+ε)Ĩ ⁻¹I _(QR,2) ⁰=(1+(δ−Δδ))(1+ε)Ĩ ⁻¹ I _(QR,2) ^(π)=(1+(δ−Δδ))(1+ε)Ĩ ₊₁

Solving for δ, Δδ and ε yields:

$\delta = {\frac{1}{4}\left\{ {\frac{I_{{QR},1}^{\pi} - I_{{QL},1}^{0}}{I_{{QR},1}^{\pi} + I_{{QL},1}^{0}} + \frac{I_{{QR},2}^{\pi} - I_{{QL},2}^{0}}{I_{{QR},2}^{\pi} + I_{{QL},2}^{0}} + \frac{I_{{QR},1}^{0} - I_{{QL},1}^{\pi}}{I_{{QR},1}^{0} + I_{{QL},1}^{\pi}} + \frac{I_{{QR},2}^{0} - I_{{QL},2}^{\pi}}{I_{{QR},1}^{0} + I_{{QL},2}^{\pi}}} \right\}\mspace{14mu}{and}}$${\Delta\delta} = {\frac{1}{4}\left\{ {\frac{I_{{QR},1}^{\pi} - I_{{QL},1}^{0}}{I_{{QR},1}^{\pi} + I_{{QL},1}^{0}} - \frac{I_{{QR},2}^{\pi} - I_{{QL},2}^{0}}{I_{{QR},2}^{\pi} + I_{{QL},2}^{0}} + \frac{I_{{QR},1}^{0} - I_{{QL},1}^{\pi}}{I_{{QR},1}^{0} + I_{{QL},1}^{\pi}} - \frac{I_{{QR},2}^{0} - I_{{QL},2}^{\pi}}{I_{{QR},2}^{0} + I_{{QL},2}^{\pi}}} \right\}\mspace{14mu}{and}}$$ɛ = \frac{\left( {I_{{QR},2}^{\pi} + I_{{QL},2}^{0}} \right) - \left( {I_{{QR},1}^{\pi} + I_{{QL},1}^{0}} \right) + \left( {I_{{QR},2}^{0} + I_{{QL},2}^{\pi}} \right) - \left( {I_{{QR},1}^{0} + I_{{QL},1}^{\pi}} \right)}{\left( {I_{{QR},2}^{\pi} + I_{{QL},2}^{0}} \right) + \left( {I_{{QR},1}^{\pi} + I_{{QL},1}^{0}} \right) + \left( {I_{{QR},2}^{0} + I_{{QL},2}^{\pi}} \right) + \left( {I_{{QR},1}^{0} + I_{{QL},1}^{\pi}} \right)}$

FIG. 10 illustrates a grating target that will be used as an example toillustrate a calibration process. The target in this example comprises apair of gratings T(−d) and T(+d) which are shown side by side in planview FIG. 10 (a) and in a composite cross-section in FIG. 10( b). Thesuffix −d and +d in this case indicates an overlay bias between twosuperimposed sets of grating lines. The individual gratings areelongated in the direction of periodicity (in this drawing, the Xdirection). Compared with the square shaped gratings shown earlier, thiselongate form allows a stronger diffraction signal to be maintained asthe area occupied by the grating is reduced to save space on thesubstrate.

The elongate form of grating is the subject of a separate patentapplication (reference P-3719, not published at the present prioritydate), which is incorporated herein by reference in its entirety. Thespot size will of course vary according to the instrument. It may have adiameter up to 100 μm, for example, or less than 50 μm, or less than 30μm. Individual grating portions may have a length (perpendicular totheir grating lines) which is less than 15 μm, or less than 10 μm. Acomposite target comprising at least four gratings may for example becontained in a circle of diameter less than 50 μm or less than 30 μm. Acomposite target comprising at least four gratings may for exampleoccupy a rectangular area on the substrate which is less than 200 μm²,or less than 150 μm². Within such a composite target, the individualgrating portions may each for example have a length greater than 6 μmand a width less than 6 μm. In this way, targets can be made smallenough to be located within device areas on the substrate, as well as inscribe lanes.

The layer stack in which the calibration target T is formed should beidentical as far as possible to the stack in which the targets fromwhich overlay is to be measured. Otherwise, the diffraction signals willbe influenced by process variables other than overlay, and the overlaymeasurement accuracy will be reduced in consequence. These other processvariables include for example the layer thicknesses of the componentgratings, CD and side wall angles. To achieve a desired level ofaccuracy, it may therefore be necessary to perform the calibrationprocess using target gratings on the actual substrate to be measured, orat least on a representative substrate of the same batch or lot. Thecalibration process will be time consuming as a result, but there willbe a throughput saving overall, when the calibration enables quickermeasurements to be performed on a number of targets on the samesubstrate or batch of substrates.

FIG. 11 illustrates the calibration process of the first implementation.Six measurement steps M1-M6 are performed on the target of FIG. 10, inwhich the intensity of one or both of the gratings (−d and +d) ismeasured under each of the illumination modes (QL, QR). Each grating maybe located at one of two positions separated in the Y direction, labeledPY=1 and PY=2. Both of these positions fit within the illumination spotsimultaneously. The legend RZ indicates the rotation of the target,either 0 or π (180°). The next row in the diagram illustratesschematically the Y position of each grating. The table at 1100illustrates the intensities measured in each step M1 etc. for eachgrating T(−d) and T(+d). Now that two gratings have been measured, thecorrection parameters can be calculated using the equations given above.Since both gratings have been measured in all eight permutations ofillumination, rotation and position, their results can be combined(averaged) to calculate the correction parameters with greater accuracy.Of course more gratings, and/or repeated measurements of the samegratings, can be used to improve statistical accuracy still further. Inprinciple, it would be sufficient for the calibration to measure one ofthe gratings at one target position for 0° and 180°, plus one at 0° andone at 180° at the two different positions, yielding a calibrationscheme for the positive or negative bias only. It is simpler, and givesmore accurate results for little more effort, to measure at three targetpositions for 0° and 180°. FIG. 11 illustrates this expanded set ofmeasurements.

Step S0 is then complete, and measurement of asymmetry in real targetgratings can then continue by repeated application of steps S1-S5, usingthe correction parameters automatically to eliminate or reduce machine-and position-dependent errors in the intensity difference of step S4.For example, the corrected intensity for a grating measured upright,with left illumination and at position PY=2 would be obtained byinverting the third equation in the left hand column above, i.e.:Ĩ ₊₁ =I _(QL,2) ⁰/((1−(δ−Δδ))(1+ε))This single calculation combines the correction steps S2 a and S2 b (orS3 a and S3 b) in the process of FIG. 9.

Note that the grating rectangle represents a region of interest (ROT).As the ROI covers a number of pixels in the sensor 23 image field, eachintensity measurement in the calibration process, as well as in theactual measurement process is an integral or average of the individualpixel intensities covered. The ROI may be trimmed smaller than the fullgrating image, for example to eliminate edge effects, if desired. Wnerethe ROI is smaller than the grating, it can also be trimmed to matchmore closely the designated position, and eliminate position-dependentinaccuracy in the measurements. Provided the same trimming is performedin the calibration steps as in the actual measurements, the accuracy ofthe results will not be affected, and may be enhanced.

Correction Implementation 2:

Referring to FIGS. 8, 9 and 12, a second implementation of thecorrection scheme will be described. The same principles and notationscan be applied (or variations of them that can be envisaged by theskilled reader). In this second implementation, instead of consideringthe corrections for the grating at only certain allowed positions,correction parameters are defined and stored for any possible positionof the target grating within the illumination spot as imaged on thesensor 23. The pixels and the illumination spot position are consideredto be fixed in relation to the optical system of the scatterometer. (Thecalibration and correction methods can be adapted still further ifeither of these assumptions is not true in a particular embodiment.)

Referring back to FIG. 8, it is recalled that the grating positions maynot coincide with predetermined positions such as those labeled P1-P4,but may lie at variable positions such as P1′ or P1″. These variationsin position may arise because the positioning system of the substrate isincapable of positioning the target more accurately, or because theoperator does not want to incur a throughput penalty by allowingsufficient time for the target to be positioned more accurately. Anadvantage of this second implementation is that possible positioningerrors of the grating can be detected, and the corrections can beapplied according to the exact (pixel) position of the grating image.Also, this may correct for artifacts such as contamination ordegradation that is locally present on the camera or at an equivalentwafer plane. In practice, as already mentioned, the calibration step S0will be repeated after a period of operation, so as to correct foradditional contamination.

In this implementation, maps of the correction parameters δ and ε aremade using a calibration measurement. The two fixed positions PY=1 andPY=2 in the first implementation, are then replaced by a finely variableposition P(i,j) of the region of interest ROI defined by an individualgrating image. Note that, in this implementation as in the firstimplementation, the size and shape of the ROI does not change. Theindices (i,j) that identify position P(i,j) can be defined by referencefor example to the top left corner of the ROI, or its centre. After thecalibration measurements have been made, the ‘ideal’ intensity value Ĩis obtained by averaging calibration measurements made over left-rightillumination, 0° and 180° degrees target rotation, as well as over allpossible positions within the spot. This yields then maps of correctionparameters δ=δ(ij), and ε=ε(i,j) that describe the deviation from theideal (average) value for each situation and for each position P(i,j),P′(i,j) etc.

As will be appreciated, these maps are stored separately for each of thedifferent illumination modes (or imaging modes, if appropriate). Inpractice, separate maps of correction parameters may also be stored foradditional wavelengths of radiation that may be used, and for differentpolarizations of radiation. If a ‘best’ wavelength is identified andused exclusively, the number of maps and the number of calibrationmeasurements can be reduced. Similarly, if the polarization known toremain constant, the number of calibration measurement and correctionparameters can be reduced.

Correction Implementation 3:

Referring to FIGS. 8, 9 and 13, a third implementation of the correctionscheme will be described. The same principles and notations can beapplied (or variations of them that can be envisaged by the skilledreader). In this second implementation, instead of considering thecorrections for the grating and its position as a whole, alternatively,the correction scheme is applied for each pixel within the illuminationspot as imaged on the sensor 23. The pixels and the illumination spotposition are considered to be fixed in relation to the optical system ofthe scatterometer. (The calibration and correction methods can beadapted still further if either of these assumptions is not true in aparticular embodiment.)

Referring back to FIG. 8, it is recalled that the grating positions maynot coincide with predetermined positions such as those labeled P1-P4,but may lie at variable positions such as P1′ or P1″. These variationsin position may arise because the positioning system of the substrate isincapable of positioning the target more accurately, or because theoperator does not want to incur a throughput penalty by allowingsufficient time for the target to be positioned more accurately. Anadvantage of this second implementation is that possible positioningerrors of the grating can be detected, and the corrections can beapplied pixel-by-pixel. Also, processing pixel-by-pixel this may correctfor artifacts such as contamination or degradation that is locallypresent on the camera or at an equivalent wafer plane. In practice, asalready mentioned, the calibration step S0 will be repeated after aperiod of operation, so as to correct for additional contamination.

In this implementation, maps of the correction parameters δ and ε aremade using a calibration measurement. The positions PY=1 and PY=2 in thefirst implementation, are then replaced by position P(i,j) of pixel(i,j). The ideal value obtained by averaging over left-rightillumination, 0° and 180° degrees target rotation, as well as over allpixels within the spot. This yields then the maps of δ=δ(i,j), andε=ε(i,j) that describe the deviation from the ideal (average) value.

As will be appreciated, these maps are stored separately for each of thedifferent illumination modes (or imaging modes, if appropriate). Inpractice, separate maps of correction parameters may also be stored foradditional wavelengths of radiation that may be used, and for differentpolarizations of radiation. If a ‘best’ wavelength is identified usedexclusively, the number of maps and the number of calibrationmeasurements can be reduced. Similarly, if the polarization known toremain constant, the number of calibration measurement and correctionparameters can be reduced.

In one embodiment of this type, the calibration procedure (step S0)consists of the accurate measurement of one large grating that fits wellwithin the illumination spot and which doesn't contain variations of thestructure over the grating (for example, no process effects so thatthere should be no variation in diffraction efficiency over pixels(i,j)). The calibration target can be for example a 40×40 μm2 gratingthat has similar gratings properties (the same stack) as the smallin-die targets that need to be measured. The measurements need to beperformed for left- and right-incident light, and for both 0° and 180°degrees wafer rotation, for the calibration in the x-direction. Similarfor the y-direction.

For this calibration step, the target may be biased. Therefore one canuse for example the X1 and Y1 gratings of a standard 40×160 μm2 overlaytarget that is positioned in the scribe lane. A special calibrationtarget is not therefore required, but a large target with propertiesthat a similar to the small targets needs to be available.

In the illustration of FIG. 13, we see schematically how measured pixelintensity values I(i,j) are corrected to ‘ideal’ values Ĩ(i,j), usingpixel correction parameters δ(i,j) and ε(i,j), that have been stored inassociation with the pixels of the image field. When it comes to applycorrection in the steps S2 a/S2 b or S3 a/S3 b, the pixel correctionparameters for each the pixels falling within the region of interest(ROI) are then retrieved and applied to the intensities measured atthose pixels, before integrating (summation E) the pixel intensitiesover the ROI. Although the amount of data storage and calculation perROI is much greater in this implementation than in the firstimplementation, it is able to maintain accuracy in the presence ofvariations of the ROI position of the type shown in FIG. 8. Anotheradvantage of this third implementation is that the same map ofcorrection parameters can be used to correct a measured image ofarbitrary size and shape. FIG. 12 shows in broken outline a secondregion of interest labeled ROI', that can be imaged and corrected usingthe same pixel correction parameters. Compared with the secondimplementation, calibration may be faster using the single largegrating. However, there may be a disadvantage in that the calibrationmeasurements are not performed using gratings identical to those thatwill pertain during the subsequent measurements. For example, it can beappreciated that the intensities recorded during the calibration processin the second implementation will include edge effects and inter-pixeleffects caused by partial coherence of the radiation which will moreclosely replicate those occurring in the subsequent measurement steps.To save processing and/or storage space, it would be possible to storethe correction parameters at a lower resolution than the image.Correction parameters might then be applied to blocks of pixels of themeasured image, or interpolated. Correction accuracy may be reduced,because of mismatch between the ROI edges and the block edges, and dueto high spatial frequency effects (e.g., dirt on a single camera pixel),but this may be tolerable in the interest of storage or computationalburden. The ROI may be trimmed to the block boundaries, if the gratingis larger than the ROI required for accurate asymmetry measurement. Ahybrid of the first and second implementations might be adopted, inwhich correction parameters are pre-stored for a number of ROI positions(like the first implementation), but then the correction parametersapplied to a measured ROI are interpolated between two or more of thecorrection ROIs, according to the overlap or proximity of their areas.

The corrections taught above enable one or more of the followingbenefits. Enhanced throughput possible of the DF-overlay measurementwhile maintaining the correctness of the measured overlay value. This isachieved by using opposite illumination angles instead of substraterotation, and/or by simultaneous readout of two or gratings at differentpositions within the illumination spot. Reduction of dependence of themeasured overlay on positional accuracy in the illumination spot.Reduction of the dependence on the incident illumination angle for theDF-overlay. Reduction of the effects of artifacts due to for exampledirt in a wafer equivalent plane. Potential to correct for aberrationdependencies, partial coherence and neighboring structures, incombination with the grating position (implementations 1 and 2).

While specific embodiments of the present invention have been describedabove, it will be appreciated that Embodiments of the present inventionmay be practiced otherwise than as described. The examples above featuremultiple measurements using dark field imaging techniques, where thezero order diffracted radiation is excluded from the optical path to theimage sensor 23. The techniques described above can also be used toapply corrections in other types of measurement. For example, in othertechniques, intensity measurements from targets may be made using zeroorder radiation to form an image. Where the target is smaller than anillumination spot and the image of a target component is smaller than afield of view of the measurement optical system, several intensitymeasurements can be performed in a single step by arranging severaltarget components in a composite target, forming an image using thedesired radiation, and separating the measurement results by detectingthe intensity in different parts of the image sensed by the sensor.Since each target component in that case is then measured through adifferent optical path, the measurements will include aposition-dependent error, in addition to the wanted variations thatrepresent process parameters to be measured.

The techniques described above can be performed to calibrate and thencorrect the intensity measurements, according to the position of theindividual components of the image. Each target component can bemeasured at each of a fixed set of possible locations as part of thecalibration process, similar to the first implementation describedabove. The calibration process can be performed for freely variablepositions, similar to the second implementation, described above withreference to FIG. 12. The calibration process can be performed forindividual pixels of the image field, so as to allow the region ofinterest to be freely defined in the manner of the third implementationdescribed above with reference to FIG. 13. If the measurement processuses different wavelengths and/or polarizations of radiation, thencalibration can be performed for these separately. On the other hand,these zero-order embodiments will not feature the different illuminationmodes and so the correction parameter 6 may be redundant.

While the target structures described above are metrology targetsspecifically designed and formed for the purposes of measurement, inother embodiments, properties may be measured on targets which arefunctional parts of devices formed on the substrate. Many devices haveregular, grating-like structures. The terms ‘target grating’ and ‘targetstructure’ as used herein do not require that the structure has beenprovided specifically for the measurement being performed.

In association with the physical grating structures of the targets asrealized on substrates and patterning devices, an embodiment may includea computer program containing one or more sequences of machine-readableinstructions describing a methods of producing targets on a substrate,measuring targets on a substrate and/or analyzing measurements to obtaininformation about a lithographic process. This computer program may beexecuted for example within unit PU in the apparatus of FIG. 3 and/orthe control unit LACU of FIG. 2. There may also be provided a datastorage medium (e.g., semiconductor memory, magnetic or optical disk)having such a computer program stored therein.

Although specific reference may have been made above to the use ofembodiments of the present invention in the context of opticallithography, it will be appreciated that embodiments of the presentinvention may be used in other applications, for example imprintlithography, and where the context allows, is not limited to opticallithography. In imprint lithography a topography in a patterning devicedefines the pattern created on a substrate. The topography of thepatterning device may be pressed into a layer of resist supplied to thesubstrate whereupon the resist is cured by applying electromagneticradiation, heat, pressure or a combination thereof. The patterningdevice is moved out of the resist leaving a pattern in it after theresist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The foregoing description of the specific embodiments will so fullyreveal the general nature of embodiments of the present invention thatothers can, by applying knowledge within the skill of the art, readilymodify and/or adapt for various applications such specific embodiments,without undue experimentation, without departing from the generalconcept of the present invention. Therefore, such adaptations andmodifications are intended to be within the meaning and range ofequivalents of the disclosed embodiments, based on the teaching andguidance presented herein. It is to be understood that the phraseologyor terminology herein is for the purpose of description by example, andnot of limitation, such that the terminology or phraseology of thepresent specification is to be interpreted by the skilled artisan inlight of the teachings and guidance.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

The present invention has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of embodiments of the present invention thatothers can, by applying knowledge within the skill of the art, readilymodify and/or adapt for various applications such specific embodiments,without undue experimentation, without departing from the generalconcept of the present invention. Therefore, such adaptations andmodifications are intended to be within the meaning and range ofequivalents of the disclosed embodiments, based on the teaching andguidance presented herein. It is to be understood that the phraseologyor terminology herein is for the purpose of description and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

The invention claimed is:
 1. A method of measuring asymmetry in aperiodic structure formed by a lithographic process on a substrate, themethod comprising: using the lithographic process to form a periodicstructure on the substrate; a first measurement step comprising formingand detecting a first image of the periodic structure while illuminatingthe structure with a first beam of radiation, the first image beingformed using a first part of non-zero order diffracted radiation whileexcluding zero order diffracted radiation; a second measurement stepcomprising forming and detecting a second image of the periodicstructure while illuminating the structure with a second beam ofradiation, the second image being formed using a second part of thenon-zero order diffracted radiation which is symmetrically opposite tothe first part, in a diffraction spectrum of the periodic structure; andusing a difference in intensity values derived from the detected firstand second images together to determine the asymmetry in the profile ofthe periodic structure, wherein the first and second measurement stepsare performed using different optical paths within a measurement opticalsystem, and at least one correction, applied either to the first andsecond image intensity values or to the calculated difference betweenthem, for reducing an influence on the determined asymmetry of thedifference in optical paths between the first and second measurementsteps.
 2. The method of claim 1, wherein: at least one correctionparameter is defined with predetermined values associated with differentavailable optical paths, and the correction comprises selecting a valuefor the correction parameter according to the optical path used in oneor both of the measurement steps and applying the correction parameterto the intensity values derived from the image obtained in thatmeasurement step.
 3. The method of claim 1, wherein: the first andsecond measurement steps are performed using respectively a first and asecond illumination mode of the measurement optical system, such thatthe first and second beams of radiation are incident on the periodicstructure from symmetrically opposed angles relative to an optical axisof the measurement optical system, without rotating the substraterelative to the measurement optical system, and at least one correctionparameter is defined to compensate for asymmetry between optical pathsdefining the first and second illumination modes.
 4. The method of claim1, wherein the first and second measurement steps are performed usingrespectively a first and a second imaging mode of the measurementoptical system such that the first and second images are formed usingportions of radiation diffracted by the periodic structure todiametrically opposed angles relative to an optical axis of themeasurement optical system, without rotating the substrate relative tothe measurement optical system, and at least one correction parameter isdefined to compensate for asymmetry between optical paths defining thefirst and second imaging modes.
 5. The method of claim 1, furthercomprising calibration wherein values for the correction parameter areat least partially determined using the results of a plurality ofcalibration measurements performed on a substrate which is rotated andmeasured without changing the optical path.
 6. The method of claim 1,wherein the correction parameter value is selected based at least inpart on the position of the periodic structure within a field of view ofthe measurement optical system.
 7. The method of claim 6 wherein: thefield of view is defined by an illumination spot of the respectiveradiation beam, the periodic structure occupies less than half the areaof the field of view such that a plurality of periodic structures can beimaged simultaneously within the image field, and a value for thecorrection parameter is defined for a set of two or more alternativepositions within the field of view at which a periodic structure may beimaged.
 8. The method of claim 1, wherein separate correction parametersare defined for correcting (a) an asymmetry in intensity betweenillumination or imaging modes used in the first and second measurementsteps and (b) a variation in illumination intensity across a field ofview of the measurement optical system.
 9. The method of claim 8,wherein the correction parameter (a) is further dependent or a positionof the periodic structure within the field of view of the measurementoptical system.
 10. An inspection apparatus configured for measuringasymmetry in a periodic structure on a substrate, the inspectionapparatus comprising: an illumination arrangement operable to deliverfirst and second beams of radiation to the substrate for use in firstand second measurements; a detection arrangement operable during thefirst and second measurements to form and detect respective first andsecond images of the substrate using radiation diffracted from thesubstrate; and a stop arrangement within the detection arrangement,wherein the illumination arrangement and stop arrangement together areeffective to stop zero order diffracted radiation contributing to thefirst and second images, and are configurable to form first and secondimages using first and second parts respectively of the non-zero orderdiffracted radiation, the first and second parts being symmetricallyopposite one another in a diffraction spectrum of the diffractedradiation, a computational arrangement operable to determine theasymmetry using a difference in intensity values derived from the firstand second images, the computational arrangement is operable whencalculating the difference to apply a correction for reducing aninfluence on the determined asymmetry of a difference between first andsecond optical paths, that are used within the inspection apparatus forthe first and second measurements respectively.
 11. The inspectionapparatus of claim 10, wherein the inspection apparatus is furtheroperable to perform a plurality of calibration measurements on asubstrate which is rotated and measured without changing the opticalpath, and wherein the computational arrangement is further operable todetermine, from results of the calibration measurements, values for thecorrection parameter.
 12. The inspection apparatus of claim 10, wherein:the detection arrangement is operable to image a plurality of periodicstructures simultaneously within the image field, and the computationalarrangement is further operable to define a value for the correction fora set of two or more alternative positions within the field of view atwhich a periodic structure may be imaged.
 13. A lithographic systemcomprising: a lithographic apparatus comprising: an illumination opticalsystem arranged to illuminate a pattern; a projection optical systemarranged to project an image of the pattern onto a substrate; and aninspection apparatus comprising, an illumination arrangement operable todeliver first and second beams of radiation to the substrate for use infirst and second measurements; a detection arrangement operable duringthe first and second measurements to form and detect respective firstand second images of the substrate using radiation diffracted from thesubstrate; and a stop arrangement within the detection arrangement,wherein the illumination arrangement and stop arrangement together areeffective to stop zero order diffracted radiation contributing to thefirst and second images, and are configurable to form first and secondimages using first and second parts respectively of the non-zero orderdiffracted radiation, the first and second parts being symmetricallyopposite one another in a diffraction spectrum of the diffractedradiation, a computational arrangement operable to determine theasymmetry using a difference in intensity values derived from the firstand second images, the computational arrangement is operable whencalculating the difference to apply a correction for reducing aninfluence on the determined asymmetry of a difference between first andsecond optical paths, that are used within the inspection apparatus forthe first and second measurements respectively, wherein the lithographicapparatus is arranged to use the measurement results from the inspectionapparatus in applying the pattern to further substrates.
 14. A method ofmanufacturing devices wherein a device pattern is applied to a series ofsubstrates using a lithographic process, the method including:inspecting at least one periodic structure formed as part of or besidethe device pattern on at least one of the substrates using an inspectionmethod comprising, using the lithographic process to form a periodicstructure on the substrate; a first measurement step comprising formingand detecting a first image of the periodic structure while illuminatingthe structure with a first beam of radiation, the first image beingformed using a first part of non-zero order diffracted radiation whileexcluding zero order diffracted radiation; a second measurement stepcomprising forming and detecting a second image of the periodicstructure while illuminating the structure with a second beam ofradiation, the second image being formed using a second part of thenon-zero order diffracted radiation which is symmetrically opposite tothe first part, in a diffraction spectrum of the periodic structure; andusing a difference in intensity values derived from the detected firstand second images together to determine the asymmetry in the profile ofthe periodic structure, wherein the first and second measurement stepsare performed using different optical paths within a measurement opticalsystem, and at least one correction, applied either to the first andsecond image intensity values or to the calculated difference betweenthem, for reducing an influence on the determined asymmetry of thedifference in optical paths between the first and second measurementsteps, and controlling the lithographic process for later substrates inaccordance with the result of the inspection method.